The immune system is critical for host defense against invasion by infectious organisms. Although the role of this system in cancer has yet to be fully defined, there has been considerable progress in understanding how cancer cells escape from this system, and also how this system can be amplified to treat cancer patients. The main focus of our research is to dissect the molecular mechanisms regulating the transition from innate to adaptive immunity and identify a new means to control this complex system. Chemokines are small molecular weight, secreted proteins that regulate the trafficking of leukocytes in both physiological and pathological conditions. Recent studies have indicated that the chemokine/chemokine receptor system plays a role in cancer development and metastasis, reinforcing the relevance of studies defining this complex system to cancer. We previously purified and cloned the human chemokine monocyte chemoattractant protein-1 (MCP-1), also known as CCL2, as a molecule potentially responsible for the recruitment of monocytes to sites of delayed-type hypersensitivity (DTH) reaction, an adaptive immune response, and also to tumors in which macrophage infiltration is commonly observed. Studies of MCP-1-deficient mice performed by others defined this molecule as a major monocyte chemoattractant in vivo. In addition, MCP-1 is reported to attract T cells, NK cells and dendritic cells (DCs), and to play a role in the replication of human immunodeficiency virus-1. Thus, MCP-1 is involved in the pathogenesis of many important human diseases, including chronic inflammatory diseases and cancer. MCP-1 is produced by multiple cell types, including macrophages, endothelial cells, epithelial cells and neutrophils. However, the particular cell type(s) that is the major source of MCP-1 in each human disease or mouse disease model remains to be determined. Macrophages and endothelial cells are known to express MCP-1 at sites of many human disease sites, such as atherosclerosis and pulmonary fibrosis. In a rat DTH model, infiltrating neutrophils were the main MCP-1 producer and blockade of MCP-1 inhibited the recruitment of monocytes and T cells and subsequent development of DTH. These previous results led us to hypothesize that different cell types contribute to the production of MCP-1 in each disease. To evaluate the role of MCP-1 produced by different cell types, we initiated the construction of conditional tissue-specific MCP-1 gene knockout mice using the Cre/loxP system. We recently obtained mice in which the MCP-1 gene is flanked with two loxP sequences, one of which contains the neo gene. We are currently in the process of removing the neo gene by intercrossing the mice with EIIaCre mice. During the process, we obtained systemic MCP-1 KO mice (MCP-1-/+) which were subsequently used to generate MCP-1+/+, MCP-1+/- and MCP-1-/- mice. Using these mice, we evaluated the role of MCP-1 in the recruitment of monocytes during thioglycollate (TG)-induced peritonitis. There was no significant difference in the number of resident macrophage between the mice with the three different genotypes and TG injection caused the recruitment of neutrophils into the peritoneal cavities of all mice at a comparable level after 4 hours. Interestingly, however, the numbers of infiltrating monocytes 24 h and 96 h after TG injection were markedly reduced in MCP-1-/- mice in compared to those in MCP-1+/+ mice. These results indicate that MCP-1, possibly neutrophil-derived, plays a critical role in the recruitment of monocytes during the inflammatory responses, such as TG-induced peritonitis, but not in the distribution of resident macrophage at steady state, supporting the results of previous studies. The production of tissue-specific conditional MCP-1 KO mice will provide a powerful tool to study the source of MCP-1 and to identify cellular targets to intervene in its production.